Known laser beam levelling devices serve to determine a horizontal or an inclined plane and are used for building purposes. The laser beam issuing from the device is received at a relatively great distance by a receiver. The inclination of the imaginary plane between the laser beam device and the receiver may therefore be measured. Laser beam levelling devices are used, for example, in ground excavation and the grading of pits, in the monitoring of concrete foundations and concrete ceilings, orientation of formwork, in the levelling of tracks for building cranes and many more.
Canal building laser devices are a subgenus of laser beam levelling devices. They are used in the construction of pipelines, for example of drainage pipes, in order to lay the individual lengths of pipe in the desired inclination and direction. The lengths of pipe are lined up until they form a pipeline with an inclination of, for example, 1.2%. Shafts are installed at intervals; there is no change of direction between the shafts. The inclination of the individual portions of pipeline between the shafts has to differ in each case in practice.
During the construction of a pipeline, the canal building laser device is erected at the beginning of a line, i.e. normally in the region of a shaft, such that the laser beam points in the desired direction and has the desired inclination. The laser beam therefore indicates both the direction and the inclination in which the pipeline is to be constructed. The receiver is inserted at the opposite end of the length of pipe to be positioned. The receiver usually has a transparent disc with markings in the form of a reticule. The laser beam issuing from the canal building laser device strikes the disc of the receiver as a spot of light. As soon as this spot of light is located in the centre of the reticule, the length of pipe is correctly positioned and may be fastened in this position.
The inventor has disclosed a laser beam receiver for canal building laser devices which has photocells arranged in the form of a cross on the side which is to face the laser beam. These photocells are opposed by light-emitting diodes on the opposite side of the receiver. The advantage of this receiver is that the laser beam may be received at a distance of up to 300 m and may easily be read off owing to the light-emitting diodes.
If work on a pipeline is shut down in the evening, the expensive laser beam devices have to be removed and stored safely. The devices have to be erected again on the following day before building work can continue. Although a canal building laser device is designed such that it may readily be orientated with respect to the longitudinal axis of the pipe during erection, it will be difficult to locate it as on the previous day to the nearest millimeter. A canal building laser device is usually provided with a spirit level which is visible from the exterior and allows the device to be erected in the pipe cross section such that it is almost orientated with respect to the longitudinal axis of the pipe. However, even the slightest deviation inevitably causes the laser beam arriving in the region of the receiver at a distance of, for example, 50 to 100 m to be located clearly next to the longitudinal axis of the pipe. As the concrete pipes normally used for producing water or drainage pipes also have a relatively rough and uneven surface, it is almost impossible to position the canal building laser device such that the laser beam issuing at the inclination and azimuth angle values input the previous day is located exactly in the longitudinal axis of the finished portion of pipeline. For this reason it is necessary to input again at least the azimuth angle or to shift the device until the direction of the laser beam matches. A drawback of this arrangement is that two people are required for this work: one who takes the receiver to the end of the pipeline and observes where the laser beam impinging as a spot of light is located and another person who remains at the canal building laser device and operates it. Problems of communication may also arise owing to the often great distance between the canal building laser device and the receiver, so radio equipment may also be required.